Process for preparing calcium thiosulfate solution

ABSTRACT

An efficient process to produce calcium thiosulfate (CaS 2 O 3 ) from lime, sulfur and oxygen is described. By selecting appropriate process conditions such as mole ratios of lime to sulfur, temperature and pressure of the reaction process and the oxidation conditions, including rate and duration, the concentration of byproducts in the resulting suspension can be reduced to about 2% by weight or less. The solid particulate dispersion in the suspension tends to form a slimy solid suspension that is hard to filter if not treated properly. The suspension then can be acidified and treated with a flocculent. This agglomerates the solids into a floc that filters with ease. The resulting calcium thiosulfate is a clear liquid with concentrations achievable up to 29%.

FIELD OF THE INVENTION

The present invention is directed to plant nutrient solutions and, moreparticularly, to plant nutrient solutions containing calciumthiosulfate.

DESCRIPTION OF RELATED ART

The thiosulfate ion, S₂O₃ ²⁻, is a structural analogue of the SO₄ ²⁻ ionin which one oxygen atom is replaced by one S atom. However, the twosulfur atoms in S₂O₃ ⁻² are not equivalent. One of the S atoms is asulfide-like sulfur atom that gives the thiosulfate its reducingproperties and complexing abilities.

Thiosulfates are used in flue-gas de-sulfurization, cement additives,dechlorination, ozone and hydrogen peroxide quenching, coatingstabilizers, and so on.

Due to complex-forming abilities with metals, thiosulfate compounds havealso been used in commercial applications such as photography, wastetreatment, and water treatment applications.

Thiosulfates readily oxidize to tetrathionates and sulfates:S₂O₃ ²⁻+O₂

S₄O₆ ²⁻→2SO₄ ²⁻

Due to this transformation, thiosulfates are used as fertilizers incombinations with cations such as ammonium, potassium, magnesium andcalcium. The ammonium, alkali metal and alkaline earth thiosulfates aresoluble in water. Water solubilities of thiosulfates decrease fromammonium to alkali metals to alkaline earth thiosulfates.

Calcium is an essential plant nutrient. Calcium availability isessential in the biochemistry of plants and, as it has been learnedrecently, in the nitrogen fertilizer efficiency of surface-applied urea.This should not be confused with the role of important soil amendmentssuch as lime or gypsum with the need of soluble calcium by high-valuecrops. Both are extremely important in soil fertility and plantnutrition and complement each other.

Calcium has been applied as foliar in apple orchards as a preventive toa physiological problem known as “bitter pit’ caused by calciumdeficiency. Calcium is also important to potato, tomato, lettuce,carrot, alfalfa and other fruit and vegetable production. Tomato plantswith calcium deficiency show severe infection with Fusarium oxysporum,the fungal pathogen that causes wilt and crown rot in tomatoes.

Although soluble calcium could be attained from calcium nitrate, calciumchloride and calcium ammonium nitrate (CAN), due to the NO₃ ⁻, or Cl⁻anion presence, more environmentally friendly counter ion such S₂O₃ ⁻²is more desirable in the fertilizer application of calcium product.

It is contemplated that calcium thiosulfate can be produced usingseveral alternative routes, such as:

-   -   I. Reaction of S and SO₃ ²⁻ in neutral or alkaline medium;    -   II. Reaction of S²⁻ and SO₃ ⁻² (via SO₂ and HSO₃ ²⁻);    -   III. Oxidation of calcium hydrosulfide (Ca(HS)₂);    -   IV. Ion exchange reaction between alkaline thiosulfates and        calcium chloride or nitrate;    -   V. Salt exchange between alkaline thiosulfates and lime, calcium        chloride or nitrate; or    -   VI. Oxidation of calcium polysulfide.

However, most of these alternatives present serious difficulties. Forexample, routes I and II suffer from low calcium sulfite solubility aswell as the need for SO₂. Route III suffers from the drawback thatcalcium hydrosulfide is unstable and decomposes to form hydrogensulfide. Route VI and V suffer from the drawbacks that ion exchange andsalt exchange require expensive raw materials and equipment, and alsorequire a step of final stripping due to the need for working withdilute solutions. It is contemplated that under appropriate conditions,Route IV should produce higher concentration calcium thiosulfatesolutions having lower byproduct concentrations compared to these otherapproaches.

Although calcium thiosulfate has been known for many years with manyreferences in the literature, there are no known commercial methods thatemploy inexpensive raw materials to produce a high purity calciumthiosulfate solution with low solid byproducts and solid residue ofinsoluble calcium salts, to provide easy separation of the desiredcalcium thiosulfate product.

Swaine, Jr. et. al. U.S. Pat. No. 4,105,754 describes the production ofcalcium thiosulfate by a metathesis reaction of ammonium thiosulfate andcalcium hydroxide or calcium oxide. This approach requires constantremoval of ammonia by air stripping at below boiling point of themixture and capturing the gas. Not all the ammonia could be removed bythis process and the resulting calcium thiosulfate could have alingering ammonia odor and or contamination of ammonia or ammoniumthiosulfate.

Japanese Patent 6,039 (1973) describes the preparation of calcium andmagnesium thiosulfate by treating sulfur and the corresponding sulfitein an alkaline solution. High yields are only obtained with magnesiumthiosulfate. This patent also described the formation of calciumthiosulfate from salt exchange process between magnesium thiosulfate andcalcium hydroxide.

Sodium thiosulfate and calcium chloride were used in the Spanish Patent245,171. The byproduct of this approach is a large amount of sodiumchloride that also contaminates the resulting calcium thiosulfate.

There remains a need for alternative processes for preparing calciumthiosulfate solutions. It would be desirable to develop an efficient andcost-effective process for preparing calcium thiosulfate solutions,especially one which can produce high concentration calcium thiosulfateproduct solutions and which can utilize relatively inexpensive startingmaterials.

BRIEF SUMMARY OF THE INVENTION

The present invention is directed to the preparation of calciumthiosulfate from calcium polysulfide (lime sulfur) oxidation. In theprocess of the present invention, a calcium hydroxide slurry isprovided. Sulfur is added to the calcium hydroxide slurry and reactedwith the calcium hydroxide to form a reaction mixture. The reactionmixture is cooled, if needed, to a temperature suitable for oxidation.An oxidant is added to the reaction mixture and reacted under conditionssufficient to form calcium thiosulfate, and the calcium thiosulfatesolution is recovered.

Liquid solutions containing high concentrations of calcium thiosulfatecan be prepared in accordance with the present invention, having onlyminimal quantities of solid byproducts and unreacted sulfur. Theconditions for oxidation, e.g., time and temperature, preferably areselected to reduce further oxidation of the thiosulfate product tosulfate.

In another aspect of the invention, a contactor/reactor apparatus isprovided for reacting lime-sulfur and an oxidant to prepare a calciumthiosulfate solution. The apparatus comprises (i) a bubble column forproviding contact between gas bubbles and liquid in a liquid/slurry;(ii) a mechanical agitator for dispersing the gas bubbles within thebubble column; (iii) a venturi ejector/eductor for ejecting accumulatedgas at the top portion of the bubble column and educing the gas througha venturi, wherein the venturi contacts the gas with a recirculatedportion of the liquid/slurry; and (iv) a pipe/tube contactor incommunication with the venturi for contacting the gas and a recirculatedportion of the liquid/slurry mixture and returning the mixture to thebottom portion of the bubble column.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described in more detail withreference to preferred embodiments of the invention, given only by wayof example, and illustrated in the accompanying drawing in which:

FIG. 1 is a graphical illustration of the filtration rate of a calciumthiosulfate slurry as a function of pH;

FIG. 2 is a graphical illustration of the percent filtrate of a calciumthiosulfate slurry as a function of pH;

FIG. 3 is a graphical illustration of the percent solid cake of acalcium thiosulfate slurry as a function of pH;

FIG. 4 is a graphical illustration of the pH stability of differentcalcium thiosulfate solutions over time;

FIG. 5 is a graphical illustration of the oxidation rate of lime-sulfuras a function of pressure;

FIG. 6 is a graphical illustration of the progression of oxidationreactions under different conditions as a function of time;

FIG. 7 is a graphical illustration of the oxidation rate of lime-sulfuras a function of temperature; and

FIG. 8 is a process flow diagram, including a schematic illustration ofa contactor/reactor, in accordance with a preferred embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention utilizes the oxidation of a slurry of lime sulfurand calcium hydroxide with oxygen for the preparation of high purity andconcentrated calcium thiosulfate without further need for concentrationby evaporation. Advantageously, calcium thiosulfate can be prepared frominexpensive raw materials, such as calcium oxide, sulfur, and oxygen. Inthe preferred practice of the invention, only minor quantities ofbyproducts are formed. These residual byproducts generated during limesulfur production and oxidation usually include calcium sulfite, calciumsulfate, and metal sulfides. Metal sulfides can form as a result ofmetal oxide impurities in the calcium oxide. In addition, calciumcarbonate and silica impurities may accompany commercial calcium oxide.These byproducts and impurities usually amount to less than 2% by weightof the final calcium thiosulfate solution when a high grade of calciumoxide is utilized and the formulation and reaction conditions areadequately controlled. Formulation and reaction conditions are chosen tomaximize the utilization of raw materials to form the calciumthiosulfate product and to minimize thermal decomposition and oxidationof the formed product to calcium sulfite and sulfate byproducts.

Residual reaction byproducts, raw material impurities, and unreacted rawmaterials in the calcium thiosulfate solution can form solid particulatematter, which preferably is filtered out in order to provide a clear,solids-free product solution. Particulate materials, such as calciumsulfate, calcium sulfite, calcium carbonate, calcium hydroxide, metalsulfides and sulfur, can be filtered more efficiently by adjusting pHand by choosing an appropriate flocculent.

The process of the invention includes a series of process steps, whichcan be implemented in equipment designed to provide the desired processconditions. The process steps can be accommodated in a single reactionvessel with the appropriate auxiliary equipment (pumps, piping, valves,heat exchangers, filters, controls, etc.) to produce one single batch ata time. The process steps alternatively can be carried out in a seriesof reaction vessels and holding tanks to facilitate a semi-continuousproduction arrangement. The process steps can be subdivided into threesections: lime slaking, reaction, and filtration, all normally operatedas batch operation (FIG. 8). However, it is contemplated that additionalequipment and controls could be used to implement continuous operation.

In the slaking section a weighed amount of dry lime and measured amountof water are mixed to produce a calcium hydroxide, or lime slurry. Thedry lime consists primarily of calcium oxide, sometimes referred to asQuicklime, which reacts with water to produce calcium hydroxide, alsoknown as hydrated lime or slaked lime. The reaction is exothermic andcauses the temperature of the lime slurry to rise. The temperature risealso assists in speeding and completing the reaction. Cooling equipmentgenerally is not required.CaO+H₂O→Ca(OH)₂+Heat

The dry lime can also consist primarily of calcium hydroxide, also knownas hydrated lime, which can be mixed with water to produce a limeslurry. In this case there is no reaction and no temperature rise. Thelime slurry could also be transferred into the production equipment asready-made lime slurry from another source and diluted with water to thedesired concentration for optimum production of calcium thiosulfate.

The calcium hydroxide concentration should be selected in accordancewith such factors as the desired calcium thiosulfate concentration inthe final solution. It was found, for example, that when an 11.7%Ca(OH)₂ solution was used, a ˜24% calcium thiosulfate solution could beprepared. A slightly higher concentration may be used to account forlosses of calcium to insoluble byproducts of calcium sulfite and calciumsulfate. Proportionately higher concentrations of lime slurry can beused to produce up to a ˜29% calcium thiosulfate solution.Concentrations of ˜30% calcium thiosulfate, as well as crystallinecalcium thiosulfate, are attainable by evaporation of the solution anddehydration of the crystals.

In the reaction section, the lime slurry transferred from the slakingsection is reacted with sulfur to produce lime-sulfur and furtherreacted with oxygen to produce calcium thiosulfate. Sulfur is added tothe lime slurry and the mixture is agitated and heated to a temperatureof about 75 to 90° C. (194° F.). Temperatures closer to 90° C. result inbetter reaction rates; however, higher temperatures result in increasedformation of insoluble calcium sulfite resulting from decomposition ofthe calcium thiosulfate. It is desirable to minimize the decompositionof calcium thiosulfate and formation of calcium sulfite as this lowersthe calcium thiosulfate produced and generates insoluble calcium solidsthat add to filtering load. Molten sulfur is added in a way such that itis dispersed into small particles or it can be added in the solid formas a ground or sublimed powder or as granules or prills. A reactionbetween sulfur and lime takes place to produce lime-sulfur slurry as amixture of various soluble products and unreacted calcium hydroxide:3Ca(OH)₂+6S→2CaS₂+CaS₂O₃+3H₂O4Ca(OH)₂+8S→2CaS₃+CaS₂O₃+3H₂O+Ca(OH)₂5Ca(OH)₂+10S→2CaS₄+CaS₂O₃ +3H ₂O+2Ca(OH)₂

Sulfur and calcium hydroxide usually are combined at a mole ratio ofsulfur-to-calcium hydroxide of from about 2:1 to 5:1, more usually fromabout 2:1 to 4:1. In one preferred embodiment, about 2 moles of sulfuris added per mole of calcium hydroxide. This is the same mole ratio ofsulfur to calcium that is in the calcium thiosulfate, CaS₂O₃ product. Ascan be seen by the above equations, this results in some carry throughof unreacted calcium hydroxide and the formation of some low solubilitycalcium disulfide along with the formation of more soluble higherpolysulfides. In the next reaction step, the lime-sulfur slurry isoxidized to produce the calcium thiosulfate product. The oxidationreaction would ideally react calcium disulfide with oxygen to producecalcium thiosulfate only:2CaS₂+CaS₂O₃ +3O₂→3CaS₂O₃

The oxidation of the lime-sulfur slurry with its contained highercalcium polysulfides, however, results the production of calciumthiosulfate and precipitated sulfur. It has been found that undercontrolled oxidation reaction conditions, as described herein, that thisprecipitated sulfur further reacts with the available unreacted calciumhydroxide to continue the lime-sulfur reaction during the oxidationreaction. The resulting product at the end of the oxidation reactionstep is the desired calcium thiosulfate product with a 2:1sulfur-to-calcium ratio and very little unreacted sulfur and or calciumhydroxide left over. Virtually all the polysulfide is oxidized tothiosulfate. It has also been found that by carrying this unreactedcalcium hydroxide in the lime-sulfur slurry into the oxidation reaction,the precipitated sulfur readily reacts with calcium hydroxide andprevents sulfur and other insoluble calcium compounds from collecting onthe equipment surfaces. This is advantageous in maintaining heatexchange efficiency and in avoiding or reducing the need to cleaninternal surfaces of the process equipment.

An alternate method is to produce lime-sulfur solution at a mole ratioof about 3.6 to 4 moles of sulfur per mole calcium hydroxide, whichresults in a much more soluble lime-sulfur solution product that is moresuitable for holding or storing for extended periods of time. This isnot, of course, the theoretical 2:1 molar ratio of sulfur to calciumthat is desirable for optimum production of calcium thiosulfate.Oxidation of this solution will result in calcium thiosulfate withexcess precipitated sulfur left over in the solution that must befiltered out and disposed of or recycled back into lime-sulfurproduction, which adds additional processing and handling.

In an alternate embodiment of the invention, a lime-sulfur solution isinitially produced at a mole ratio of about 3.6 to 4 moles of sulfur permole calcium hydroxide, and then the amount of calcium hydroxiderequired to obtain the stoichiometric 2:1 ratio is added prior to orduring the oxidation step.

The oxidation step can be carried out in the same reaction vessel as forthe lime-sulfur production or can be carried out in separate processequipment. The lime-sulfur slurry is cooled to the preferred oxidationtemperature and reacted with oxygen to produce calcium thiosulfate.Oxidation temperatures above 75° C. should be avoided when producingcalcium thiosulfate solutions of 24% because of calcium thiosulfatedecomposition losses. Lower oxidation reaction temperatures work butincrease the time to complete the oxidation reaction.

The oxidation reaction is exothermic and the heat generated during theoxidation process is removed with heat exchange to maintain the desiredtemperature. When all the lime-sulfur slurry is fully oxidized tocalcium thiosulfate, no more oxygen is consumed.

The oxygen used for the purpose of oxidizing can be supplied byatmospheric air or by an enriched oxygen supply source. It is deliveredto the oxidation reactor at the desired pressure and volume required tosupport the oxidation reaction. The primary factors that determine therate of oxidation and the time to complete the oxidation reaction areoxygen concentration, lime-sulfur slurry contact area with the oxygen,and reaction temperature. The objective is to complete the reaction in areasonable amount of time consistent with production requirements and toavoid prolonged reaction times that can lead to increased amounts ofdecomposition products and oxidation to form calcium sulfate. Whileatmospheric air or enriched oxygen is preferred, it is possible to useother oxidants, such as SO₂, with appropriate reaction modifications.

Oxygen supplied by air at atmospheric pressure is low in concentrationmaking for very long reaction times that are not suited for production.The concentration of oxygen in air can however be increased bycompression to higher pressures. Increasing its pressure to fiveatmospheres or about 60 psig increases its concentration to about thesame level as pure oxygen. When air is used the inert gases must bevented or purged periodically. Alternatively, pure oxygen can be usedadvantageously at lower pressures and without purging of inert gases.

An important consideration in maintaining good oxidation rates is toprovide efficient gas/liquid contacting that provides adequate contactarea and contact time for the oxygen carrying gas and the liquidlime-sulfur slurry to react. Contacting is important because thereaction primarily takes place at the oxygen gas liquid slurryinterface. If this interface area is not adequate, the reaction will beslow leading to larger amounts of undesirable byproducts.

Many common types of gas/liquid contacting process equipmentarrangements can be utilized for contacting and reacting the gas andliquid. These include, but are not limited to bubble columns, packedcolumns, tray columns, spray columns, mechanically agitated tanks, jetloop, venturi ejector/eductor, pipes/tubes and motionless mixers. In oneembodiment of the invention, a special contactor uses elements of commontypes of contacting equipment combined and arranged advantageously intoa single contactor-reactor design. The elements of this arrangement worktogether in a way to maximize contact time, area, and overallmass-transfer coefficient while reacting 100% of the lime-sulfur slurryand pure oxygen with recycled liquid and gas.

With reference to FIG. 8, a contactor/reactor 60 in accordance with apreferred embodiment of the present invention comprises: (i) a bubblecolumn 61, in which gas bubbles up through and contacts the liquid; (ii)a mechanical agitator 58, which further disperses bubbles within thebubble column 61 to provide additional contact area time andmass-transfer; (iii) a venturi ejector/eductor, where accumulated gas atthe top portion 61 a of the column is ejected via line 11 and educedthrough a venturi 64, where it is contacted with recirculatedliquid/slurry 10′; and (iv) a pipe/tube contactor 66 where thegas/liquid mixture exiting the venturi 64 is further contacted as it isconducted inside a draft tube, back to the bottom portion 61 b of thebubble column 61, where it is recombined with the liquid/slurry.

The resulting calcium thiosulfate product contains a small amount ofunreacted sulfur (usually about 1% or less), residual unreacted limethat is even less as long as there is a slight excess of sulfur, limeimpurities as very fine particles and a small amount of calcium sulfiteand calcium sulfate byproducts that must be separated by filtration.

The optimal lime-sulfur slurry for preparing about a 24% calciumthiosulfate will contain enough active calcium from calcium hydroxide tocorrespond to about 25% calcium thiosulfate—this is about 6.6% Ca⁺⁺. Thecontained sulfur corresponds to two times the stoichiometric amount ofcalcium to correspond to about 25% calcium thiosulfate—this is about10.5% S.

The optimum reaction conditions of time and temperature for theproduction of lime-sulfur slurry were investigated. One concern is thestability of calcium thio sulfate at near-boiling temperatures.

It is advantageous to conduct the lime sulfur synthesis in the shortesttime possible, which enhances the rate of production, and decreasesdecomposition of the calcium thiosulfate portion of the product. Ifcalcium thiosulfate decomposes to CaSO₃, it cannot be recycled in theprocess and will increase the solid byproduct. The objective was todefine the point where Ca⁺⁺ concentration was maximized. Maximum solubleCa⁺⁺ concentration indicates maximum reaction of the calcium hydroxidewith sulfur to produce soluble calcium polysulfide and calciumthiosulfate. Ca⁺⁺ concentration would serve as the defining controlparameter in this investigation process. The final lime sulfurintermediate is a thin slurry. Filtering sampled slurry and performing atitration with EDTA was done to easily and quickly monitor Ca⁺⁺.

Procedures were varied to optimize the time required to maximize calciumconcentration in lime-sulfur. In all cases, raw materials consisted ofcommercial CaO and prilled sulfur and S: Ca⁺⁺ mole ratio of 2:1. Theoptimum temperature was found to be about 90–92° C. Calcium thiosulfate,in pure solution, was determined to decompose at 97° C. Elevated H₂Sevolution is also noted at temperatures greater than 90° C. Foamingoccurs during lime-sulfur syntheses conducted near the boiling point butis not as apparent at slightly lower temperatures. Slaking of CaO aloneincreases temperature of the initial raw materials to about 50–60° C.The Ca⁺⁺ concentration stabilized after about 135 to 190 minutes atabout 90–92° C.

The next step of the process involves oxidation of lime-sulfur to thedesired calcium thiosulfate. The oxygen source could be, for example,atmospheric air or purified oxygen. Additional factors affecting theoxidation reaction include temperature, pressure and contacting area.

Oxidation of sulfides using air as the O₂ source has been mentioned forthe recovery of thiosulfates from alkali wastes of Leblanc Soda processwhere calcium hydrosulfide was oxidized to calcium thiosulfate. Theoxidation reaction was conducted within the temperature range of about55 to 65° C. The process was inefficient and only about 13% totalproduct was obtained after an extended time of purging air into thesolution at ambient pressure. Oxidation was successfully carried outwith air at elevated pressure in a lab scale pressure reactor (stirredautoclave). The reaction rate was found to be equivalent to that of pureoxygen when the partial pressure of the contained oxygen component wasthe same as the total pressure for pure oxygen. It was necessary tovent/purge out the inert nitrogen as more air was added.

Oxidation with pure 100% oxygen was successfully conducted at ambientpressure up to about 16 atmospheres and at temperatures from about 35 to75° C., resulting in complete reactions to produce high concentrationsof calcium thiosulfate. The rate of oxidation, however, was lessefficient at low temperature unless conducted at high pressure or withan enhanced contact area. Oxidation rates acceptable for preferredcommercial production were found in the about 55 to 75° C. temperaturerange at a total pressure of about one atmosphere (0.0 psig), providedactive contacting was employed to maintain a high contact area. Apressure up to about 8 atmospheres preferably is used when only nominalcontacting occurs. This active contacting was achieved and carried outin a specially designed contactor-reactor. The contactor-reactor design,as previously described, was developed for this oxidation reactionprocess to produce commercial quantities of calcium thiosulfate. Smallerquantities were successfully produced in a laboratory autoclave undersimilar conditions of temperature, pressure and gas contact area toliquid volume ratios but with the capability of exploring higherpressures. High pressures were not necessary as long as good contactingwas achieved at optimum reaction temperatures using pure oxygen. Anequivalent oxidation reaction rate can be obtained using air instead ofpure oxygen, for example, by employing pressures of at least about fivetimes that used for pure oxygen.

The oxidation reaction rate was mostly independent of the lime sulfurconcentration. This was obvious from a steady oxidation reaction ratefrom beginning to the near the end of the oxidation reaction (FIG. 6).This indicated that the polysulfide was present in large excessthroughout the reaction up to near the end of the reaction and oxygen isthe rate-limiting agent. Oxygen was added in small addition steps enoughto maintain the desired pressure. The reaction was completed when nomore oxygen was being added and no decline in pressure was observed. Thecontacting and oxidation of lime-sulfur slurry takes place primarily atthe interface of liquid-slurry/gas interface and, to a lesser degree, bythe dissolved oxygen in the lime-sulfur solution.

An increase in the reaction temperature did increase the rate ofoxidation, especially for the higher-pressure cases, but not as much aswould be expected (FIGS. 5 and 7). In general, a 10° C. temperatureincrease should increase the rate of reaction by 100%. This is likelydue to the lower gas solubility in the liquid at higher temperatures.The upper reaction temperature is limited primarily by the thermalstability of calcium thiosulfate in concentrated solutions. During theend of the oxidation reaction, when the calcium thiosulfateconcentration is reaching its maximum, it is especially vulnerable todecomposition at elevated temperatures. Calcium thiosulfate is unstableat higher temperatures decomposing primarily to calcium sulfite andsulfur. This was observed to occur with significant loss of calciumthiosulfate assay and yield at 85° C. with the formation of largeamounts of calcium sulfite and calcium sulfate. This higher oxidationreaction temperature potentially could directly oxidize the formedcalcium thiosulfate to form calcium sulfate however our observationsindicate calcium thiosulfate solution is quite resistant to oxidation,otherwise there would be significant losses during the normal oxidationreaction. Formations of significant amounts of calcium sulfate wereencountered at 85° C. but it is most likely formed as a result oroxidation of the calcium sulfite from the thermal decomposition of thecalcium thiosulfate.

Filtration of calcium thiosulfate slurry produced from the oxidationreaction was investigated thoroughly for an efficient filtration method.Fine suspensions of calcium salts such as sulfate, sulfite, hydroxideand suspended sulfur, metal hydroxides and metal sulfides generally aredifficult to filter in an efficient manner. Flocculants and coagulantshave been used in conjunction with filter aids for hard to filterslurries. There is no comprehensive quantitative theory for predictingthe behavior of these materials that can be used for their selection.This must ultimately be determined experimentally. Different anionic andnon-anionic flocculants were used for efficient filtration of theresulting calcium thiosulfate slurry.

Filtration studies were carried out using diatomaceous earth forpre-coating. Quantity of diatomaceous earth was 0.125% of the slurry.

Application of flocculants was studied over a range of temperatures. Theobjective was to verify that the temperature of the slurry did notcompromise flocculent performance. Flocculant dosage was varied attemperatures from 10–50° C. until flocculant appearance was consistent.It was found that increasing the temperature enhanced the filtrationrate.

Dosage of flocculant was also investigated from under dosing tooverdosing. Generally, the best performance was achieved when about50–70 μg/g of flocculant-to-slurry was used. Flocculant size does notappear to significantly affect filtration rate. Anionic flocculantsgenerally perform better than non-anionic flocculants.

The effect of pH on the flocculant (floc.) performance and ease offiltration of the slurry was also studied. It was observed that anionicflocculants lose their effectiveness at about pH>11. Evaluation ofcalcium thiosulfate slurries treated with an anionic flocculent, AE874,and slurries not treated were performed at pH values ranging from 6.0 to10.5. Evaluation parameters included filtration rate, relative settlingafter consistent time and % solid cake and % filtrate in comparison tooriginal slurry weight. The data is evaluated in Table 1 and furtherplotted in FIGS. 1, 2 and 3.

TABLE 1 Effects of pH Adjustment on the Filtration of CalciumThiosulfate rate of filtration % filtrate (per total slurry % solid cake(per total pH of (gm/min) weight) slurry weight) slurry control floc.control floc. control floc. 10.5 4.49 10.41 90.5 91.8 9.0 8.7 10.0 6.149.01 90.2 91.8 8.8 8.1 9.0 9.87 15.54 91.8 92.5 8.1 8.6 8.5 16.43 20.2292.9 94.3 7.1 7.6 8.0 17.71 19.18 93.2 96.1 7.5 7.0 7.5 22.33 14.88 94.794.7 7.3 7.3 7.0 15.14 17.70 94.4 97.1 6.0 5.9 6.0 17.81 22.34 96.4 97.34.9 5.5

Data indicates filtration rate is improved for both the untreatedslurries and the slurries treated with flocculent as pH is reduced.However, the rate of filtration in flocculant-treated solutions stillsurpasses that of untreated mixtures. Optimum pH is indicated betweenabout 8 and 8.5. (For the slurry control at pH=10.5, rate=4.5 gm/min;the control at pH=8.5, rate=16.4 g/min and flocculant treated slurry atpH=8.5, rate=20.2 g/min.). Data also indicates that as pH is reduced,the quantity of filtrate increases and the quantity of filter cakesolids decreases, relative to the amount of the slurry treated.

Different acids were tested for pH adjustment ranging from strongmineral acids to citric acid, and acetic acid. Calcium thiosulfate has avery low buffer capacity and it requires very small amount of acid tochange its pH. Generally, strong mineral acid tends to decompose calciumthiosulfate and easy to over shoot the pH. Citric acid and acetic acidare preferred.

Calcium thiosulfate at neutral or close to neutral pH (e.g., 7.5 to 8.5)is very stable and has a long shelf life.

Long-term stability calcium thiosulfate was addressed. Stability studieswere conducted at 40° C. for one week. All concentrations of calciumthiosulfate retained stability over this period at 40° C. in closedbottles. Calcium thiosulfate is exceedingly stable when stored in indoorconditions. An assay of a sample solution after many months did notchange.

As is noted graphically, the pH of the acid adjusted solutions remainedstable, while pH of the control (unadjusted) solution was dropped (FIG.4). pH was 10.27 after 217 days of storage. At 265 days, pH was 8.22,while the pH of an adjusted sample remained stable.

1. Lime Slaking Reaction

With reference to FIG. 8, the formulated amount of water is added to thelime slaking tank 50 through line 2, and agitation is supplied by arecirculating pump 42, which recirculates the slurry mixture into thelime slaking tank 50 via recirculating line 8. The formulated amount ofcalcium oxide is added via screw conveyor (not shown) through line 1into the water. The minimum slaking time needed is about 30 minutes. Theexothermic reaction will cause the slurry mixture to rise in temperatureby about 22° C. (40° F.).

2. Lime-Sulfur Reaction

The slaked lime is transferred to the contactor/reactor 60 through line9. The formulated amount of sulfur is added to the contactor/reactor 60through line 3. The reaction mixture can be heated to a suitablereaction temperature, e.g., at least about 70° C., and preferably about90° C. (194° F.). The reaction mixture is agitated using an impeller 58and reacted at about 90° C. for about 3 hours. At the end of thereaction, all the sulfur should be fully reacted and the calciumconcentration in the lime-sulfur solution at a maximum. The lime-sulfuris thin slurry at this point and will form a large mass of soft needlecrystals if allowed to cool to room temperature.

3. Oxidation Reaction

The lime-sulfur slurry is then cooled to the oxidation reactiontemperature of about 55–75° C., preferably about 65° C., prior to theintroduction of oxygen. The vapor space is evacuated or purged outthrough line 14 with oxygen in order to have the maximum oxygenconcentration for operation at minimum reactor pressure.

The oxidation reaction is initiated with the start of contacting byactivating the circulation pump and agitator 52. Oxygen is supplied tothe reactor 60 at a flow rate sufficient to maintain a positive reactorpressure. The heat generated by the oxidation reaction is removed by aheat exchanger 54 in order to maintain the desired reaction temperature.Preferably, the cooling is sufficient to keep the reaction temperaturefrom increasing above the set operating temperature.

The oxidation reaction generally is continued until the oxygen flowrequired to maintain reactor pressure drops off to zero. The reactionmixture will consume no more oxygen and no further heat is generated. Atthis point, substantially all of the polysulfide is converted to calciumthiosulfate and only minor amounts of calcium hydroxide and or sulfurremain unreacted, along with minor amounts of calcium sulfite and/orcalcium sulfate, which may be present as impurities associated with limefeed stock.

4. Filtering

The calcium thiosulfate product is transferred to the filter feed tank70 through line 13. The required amount of acid and flocculent are addedthrough line 5 to obtain optimum product pH and filtering properties.This treatment produces a colloidal solution, which is fed to a filter80 via pump 62. The filter 80 separates the calcium thiosulfate solution6 from the residue 7, which can be discarded or further processed. Theproduct calcium thiosulfate solution 6 is a clear liquid, withconcentrations achievable up to about 29%.

EXAMPLE 1 Lime Slaking, Bench Scale

157 grams of water is placed in a stirred reactor fitted with athermometer and 21 grams of commercial CaO is charged into the reactor.The exothermic mixture is stirred for 30–40 minutes for completeslaking.

EXAMPLE 2 Lime Sulfur Preparation

24 grams of sulfur is charged into the stirred, slacked slurry preparedin Example 1. The stirred mixture is heated to 90° C. The heating andstirring is continued for about 3 hours until all the sulfur is fullyreacted and a lime-sulfur slurry has formed.

EXAMPLE 3 Calcium Thiosulfate Preparation

The lime-sulfur slurry prepared in Example 2 is transferred to a stirredreactor capable of being pressurized and equipped with an inlet andoutlet for air purge and introduction of oxygen, a thermometer, and acooling system. Moderate stirring is applied to the mixture to providean even interface of liquid-gas and with no vortex formation. Thereactor is purged with oxygen and pressurized to 10–15 psig followed byventing to 0.0 psig. The mixture is heated to 55–75° C. Oxygen isintroduced to the reactor to initiate oxidation. Oxidation pressure ofthe system is maintained at 4–8 atmospheres. Oxidation is continueduntil oxygen is no longer absorbed, which is apparent by the absence offurther pressure drop or heat rise.

EXAMPLE 4 Filtration

The solution prepared in Example 3 is carefully adjusted to a pH of 7.5to 8.5 in the filtering tank equipped with agitation and pH electrodewith acetic acid. Filtration aid and 20–40 ppm of flocculent is added,and the mixture is filtered. The resulting calcium thiosulfate productis a colorless, odorless liquid. Concentration of nearly 27% can beprepared without significant loss of product to decomposition solids andwithout concentrating the product by evaporation.

The principles, preferred embodiments and modes of operation of thepresent invention have been described in the foregoing specification.The invention which is intended to be protected herein, however, is notto be construed as limited to the particular forms disclosed, since theyare to be regarded as illustrative rather than restrictive. Variationsand changes may be made by those skilled in the art without departingfrom the spirit of the invention.

1. A process for preparing a calcium thiosulfate solution comprising:providing a calcium hydroxide slurry; adding sulfur to the calciumhydroxide slurry at a sulfur-to-calcium hydroxide mole ratio of fromabout 3.6:1 to about 5:1 and reacting the sulfur and calcium hydroxideat a temperature of at least about 90° C. to form a reaction mixturecontaining at least one calcium polysulfide; cooling the reactionmixture to a temperature of from about 55 to about 75° C. for oxidation;adding to the reaction mixture an oxidant and reacting under conditionssufficient to form calcium thiosulfate; and recovering the calciumthiosulfate solution.
 2. The process of claim 1 further comprising aninitial step of preparing said calcium hydroxide slurry by combiningcalcium oxide and water.
 3. The process of claim 1 wherein saidsulfur-to-calcium hydroxide mole ratio is from about 3.6:1 to about 4:1.4. The process of claim 1 wherein said sulfur-to-calcium hydroxide moleratio is about 4:1 to about 5:1.
 5. The process of claim 1 wherein theoxidation reaction is conducted at a pressure of from about 10 to about15 psig.
 6. A process for preparing a calcium thiosulfate solutioncomprising: preparing a calcium hydroxide slurry by combining calciumoxide and water; adding sulfur to the calcium hydroxide slurry andreacting the sulfur and calcium hydroxide to form a reaction mixture,wherein from about 3.6 to about 4 moles of sulfur is added per mole ofcalcium hydroxide; cooling the reaction mixture, if needed, to atemperature suitable for oxidation; adding additional calcium hydroxideto the reaction mixture such that the mole ratio of sulfur to calciumhydroxide is reduced to about 2:1; adding to the reaction mixture anoxidant and reacting under conditions sufficient to form a suspensioncontaining calcium thiosulfate; adding an acid to the suspension; andadding a flocculant to the suspension and recovering said calciumthiosulfate solution.
 7. The process of claim 6 wherein said additionalcalcium hydroxide is added prior to reacting with said oxidant.
 8. Theprocess of claim 6 wherein said additional calcium hydroxide is addedduring reacting with said oxidant.
 9. The process of claim 6 wherein thestep of adding sulfur to the calcium hydroxide solution furthercomprises heating the reaction mixture to a temperature of at leastabout 55° C.
 10. The process of claim 9 wherein the reaction mixture isheated to a temperature of at least about 75° C.
 11. The process ofclaim 6 wherein the oxidation reaction is conducted at a temperature offrom about 55 to about 75° C.
 12. The process of claim 6 wherein theoxidation reaction is conducted at a pressure of from about 10 to about15 psig.
 13. A process for preparing a calcium thiosulfate solutioncomprising: providing a calcium hydroxide slurry; adding sulfur to thecalcium hydroxide slurry at a sulfur-to-calcium hydroxide mole ratio offrom about 2:1 to about 5:1 and reacting the sulfur and calciumhydroxide at a temperature of at least about 90° C. to form a reactionmixture containing at least one calcium polysulfide; cooling thereaction mixture to a temperature of from about 55 to about 75° C. foroxidation; adding to the reaction mixture an oxidant and reacting underconditions sufficient to form calcium thiosulfate; and adding aflocculant, filtering, and recovering the calcium thiosulfate solution.14. The process of claim 13 further comprising an initial step ofpreparing said calcium hydroxide slurry by combining calcium oxide andwater.
 15. The process of claim 13 wherein said sulfur-to-calciumhydroxide mole ratio is from about 3.6:1 to about 4:1.
 16. The processof claim 13 wherein said sulfur-to-calcium hydroxide mole ratio is fromabout 4:1 to about 5:1.
 17. The process of claim 13 wherein theoxidation reaction is conducted at a pressure of from about 10 to about15 psig.